WASHINGTON (CNN) --Major U.S.-led fighting in Iraq appears to be over, although some smaller confrontations continue, Pentagon officials said Monday.

"I would anticipate that the major combat engagements are over," Maj. Gen. Stanley McChrystal told reporters at a Pentagon briefing. He said U.S. forces are moving into a phase of "smaller, albeit sharper fights."

McChrystal said the number of daily air missions had dropped to 700 or 800 in recent days, down from about 1,000 or more a day. And Monday, he said, marked the last day that aircraft from all five aircraft carriers would fly missions over Iraq.

Plans were announced to scale back the American naval presence in the Persian Gulf by bringing home two carrier groups in the coming days.

The USS Kitty Hawk and USS Constellation could leave the Persian Gulf in the next several days, officials said. That would leave the USS Nimitz, which recently replaced the USS Abraham Lincoln, as the only remaining carrier group in the gulf.

Officials also said one of the two groups in the Red Sea, the USS Harry S. Truman or the USS Theodore Roosevelt, could depart soon.

Coalition fighting Monday against Iraqi targets in Tikrit, Saddam Hussein's hometown, was not as fierce as some expected. McChrystal said the Iraqi forces there lacked a "coherent defense."

Still, he cautioned against any suggestion that danger had passed for coalition forces.

Meanwhile, U.S. troops believe they have found 11 mobile chemical and biological laboratories buried south of Baghdad outside Karbala, a U.S. general said Monday. (Full story)

No chemical or biological weapons were found along with the labs, but soldiers recovered "about 1,000 pounds" of documents inside them, said Brig. Gen. Benjamin Freakley of the Army's 101st Airborne Division.

U.N. weapons inspectors in February "found nothing untoward" at an ammunition filling plant close to where the United States says troops have found the labs, a U.N. inspection team spokesman said Monday.

In February, U.S. Secretary of State Colin Powell told the U.N. Security Council that Iraq had mobile biological weapons labs on at least 18 flatbed trucks.

Disarming Iraq of its alleged weapons of mass destruction had been among the coalition's foremost goals entering the war, officials said. To date, there have been no confirmed discoveries of any such weaponry.

The "only significant combat action" Monday took place in Tikrit, the lone major Iraqi holdout, said Brig. Gen. Vincent Brooks of U.S. Central Command.

After heavy airstrikes and sporadic battles Sunday, coalition forces from the south, west and north moved early Monday into the center of the north-central Iraqi city, the hometown of the deposed Iraqi president.

The Marines met "lighter than ... expected" resistance, "isolating" Republican Guard units and taking over one of Saddam's largest and most elaborate palaces, Brooks said. (Full story)

By day's end, many Marines had switched to cleanup mode, setting up checkpoints and patrolling the city and its suburbs, said Time magazine reporter Michael Ware. Ware said people inside the city told him the vast majority of Saddam's family and forces left Tikrit three days ago.

In a further sign that the war's major fighting apparently is over, Pentagon sources said 20,000 members of the 1st Cavalry Division are not expected to deploy to Iraq in the immediate future. Similarly, only part of the 20,000-member 1st Armored Division, based in Germany, will deploy to the region.

U.S., Britain warn Syria

U.S. and British officials turned up the diplomatic heat Monday on Syria, with the White House continuing to accuse Iraq's neighbor of developing chemical weapons and warning it not to harbor members of Saddam's regime.

Powell called on Syrian leaders to "review their actions and their behavior, not only with respect to who gets haven in Syria and weapons of mass destruction, but especially the support of terrorist activity." (Full story)

Calling Syria a "state that sponsors terrorism," White House press secretary Ari Fleischer quoted from a 2002 CIA report that Syria "already held nerve gas ... but is trying develop more toxic and persistent nerve agents."

"There are acceptable standards of behavior that the world and certainly the free Iraqi people hope will be followed by its neighbors, including Syria," Fleischer said, "and part of that is not to harbor Iraqi leaders."

British Foreign Secretary Jack Straw said Damascus needed to show it is cooperating with the coalition's efforts to find Saddam and other Iraqi fugitives as well as ease concerns over its weapons program. (Full story)

Other developments

• Seven American prisoners of war rescued by U.S. Marines on Sunday said they were treated fairly well by their Iraqi captors but lived in constant fear they would be killed, Peter Baker from The Washington Post reported Monday. Baker traveled with the freed soldiers from a U.S. air base in Iraq to Kuwait. (Full story)

• British Prime Minister Tony Blair promised Monday "to make the peace worth the war" by rebuilding a democratic Iraq. Blair said that investigations have begun at seven of the 146 sites known possibly to house weapons of mass destruction in Iraq. Progress in finding such weapons is "bound to be slow," he told the House of Commons. (Full story)

• A Marine AH-1 Cobra helicopter crashed Monday near Samarra, a town about 25 miles (40 kilometers) south of Tikrit and 75 miles (120 kilometers) north of Baghdad. The pilot and co-pilot suffered minor injuries and were rescued. The incident was not the result of hostile fire, U.S. Central Command said.

• Kirkuk's water and electricity systems are "severely disrupted," and looters "seriously damaged and disrupted" health facilities, a World Health Organization team said Monday. Two hospitals in the northern Iraqi city are operating at about 25 percent of normal capacity, the WHO team said, adding that such problems are "repeated across" Iraq.

• Holed up in a central Baghdad building, gunmen battled Marines for nearly two hours Monday until U.S. forces took three men into custody whom they suspect of initiating the firefight. Marines said the men were guards at a nearby club and had been hiding near a gas station in Firdos Square, where U.S. forces helped a crowd of Iraqis topple a 40-foot statue of Saddam last week. (Full story)

• Coalition forces are hunting down fighters who have come in from outside Iraq, Brooks said Monday. Among the non-Iraqi fighters, the "greatest density" are Syrians, Brooks said, but other nationalities have been represented as well. These fighters have played a variety of roles, joining Iraqi units and ambushing coalition troops in groups of 10 to 20, he said. Brooks said Central Command believes some of these fighters have explosive vests that could be used in suicide bombings.

• Coalition forces have secured several Iraqi hospitals in recent days, transferring patients in substandard places to other Iraqi hospitals and coalition medical facilities, Brooks said Monday. More than 100 noncombatants, including Iraqi civilians, are receiving health care aboard the USNS Comfort, a U.S. military medical ship in the Persian Gulf, according to Brooks.

• Spain ordered seven Iraqi officials to leave the country by Wednesday after 21 guns and 800 rounds of ammunition were found at the Iraqi Embassy in Madrid, Spain's Foreign Ministry said Monday. Two other Iraqi diplomats -- including one who told Spain last week about the weapons cache -- will not be expelled, and the embassy will remain open, the Foreign Ministry said. (Full story)

• U.S. Navy Capt. Frank Thorp, a Central Command spokesman, confirmed Monday that the United States had two officials in custody who appear on the its 55 most-wanted list of Iraq's leaders, but he left it up in the air as to whether or not more had been captured by coalition forces. Thorp said Central Command would not make an announcement every time someone on the list was captured. (55 most-wanted Iraqis)

• Saddam's half-brother, Watban Ibrahim Hasan al-Tikriti, is in U.S. custody after attempting to escape to Syria from Iraq, a U.S. official said Sunday. Al-Tikriti was on the list of 55 most-wanted Iraqi leaders.

• U.N. relief agencies expect to return to Iraq by Monday to resume humanitarian efforts. The United Nations pulled its staff out of the country March 18. (Full story)